Liquid storage tank using metal ring band instead of thicker lower courses



J. s. ENnlco'rT ET AL oct. 7, 1969 y LIQUID STORAGE TANK USING METAL RING BAND INSTEAD OF THICKER LOWER COURSES 3 Sheets-Sheet 1 Filed March 4., 1968 .Och 7., 1969 v s, END|CQTT E'l' AL I 3,471,053

LIQUID STORAGE TNK USING METAL RING BAND INSTEAD OF v v THICKER LOWER COURSES Filed March 4, 1968 3 Sheets-Sheet 2 Oct. 7, 1969 LIQUID STORAGE 'r Filed March 4. 1968 J. S. ENDICOTT ET AL ANK USING METADRING BAND INSTEAD OF THICKER LOWER COURSES 3 Sheets-Sheet 5 Av-freeway:

United States Patent O 3,471,053 LIQUID STORAGE TANK USING METAL RING BAND INSTEAD OF THICKER LOWER COURSES John Seward Endicott, North Riverside, and Leonard Paul Zick, Jr., Hinsdale, Ill., assignors to Chicago Bridge & Iron Company, Oak Brook, Ill., a corporation of Illinois Filed Mar. 4, 1968, Ser. No. 710,243

Int. Cl. B65d 7/48 U.S. Cl. 220-5 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to large storage tanks. More particularly, this invention is concerned with an improved upright cylindrical tank having a closed bottom.

Many liquids and solids are stored at atmospheric pressure in large upright cylindrical metal tanks. The tanks have a vertical shell positioned circularly about a vertical axis and generally are made of metal plates. The lower edge of the shell is joined to a bottom, usually of metal plates, which fully encloses the tank. The top of the tank can be open or closed such as by a cone roof or floating roof.

The cylindrical shells of large diameter tanks are normally constructed of a series of horizontal rings or courses of metal plate. The edges of adjoining courses are joined, usually by girth seam butt welds. The height of each course can vary but usually will be about eight to ten feet high. Each course is of uniform thickness but the courses are successfully graduated in decreasing thickness from the bottom course to the top course of the tank. The uppermost course usually has some specied minimum thickness required for general stability of the tank shell or wall. The thickness of each course is determined from the allowable stress of the metal used, the ldensity of the material to be stored in the tank, the diameter of the tank and the height of material to be stored in the tank measured from the top of the tank to the bottom of the course less one foot. The specifications for producing such tanks are well known in the art.

When a large diameter tank produced according to standard procedures is lled with a material to the design limit, the cylindrical shell can grow considerably, except at the bottom where growth increases an insignificant amount. This causes the cylindrical shell to form a curved shape or transition between the restraining bottom and the freely expanding upper portions. Even though the restraining action of the bottom has been understood, very little practical use of this action has been made previously in building large cylindrical tanks.

It has been found according to this invention that by placing a narrow circumferential-stress resisting metal band around and in contact with the lower portion of a shell of a cylindrical metal tank that the thickness of the bottom portion of the shell can have a thickness less than required to meet circumferential-stress requirements predetermined for the maximum load to be stored in the tank without the circumferential-stress resisting metal band. Furthermore, the weight of the metal band is less than the difference between the weight of the thinner shell 3,471,053 Patented Oct. 7, 1969 lCe employing the band and the weight of a shell of the same metal required to withstand an equivalent predetermined circumferentialstress requirement without the band.

The weight of the metal band is usually at least 20% less than the said difference in weight and by careful sizing and positioning of the band can even be up to about 50-7\0% less than the difference between the banded shell and an unbanded shell designed for equivalent stresses. This permits use of less metal and the use of thinner plates which are often more readily available and less expensive than heavier plates.

In addition, the new tank structure permits construction of large diameter tanks using the same design stress with plates of the same thickness and material as would be used for small diameter tanks having no band. Thus, the invention has the advantage that the shell for larger tanks can often be reduced to a thickness which avoids the need to stress relieve the joints after welding.

The invention has its greatest use at present in storage tanks having a circular metal bottom and a vertical cylindrical shell extending around the bottom and joined thereto, said shell comprising a series of metal courses or rings of the same diameter positioned vertically with the upper courses Seton the upper edge of and joined at its bottom edge to the course below it, each course being of uniform thickness for its height and circumference and a narrow circumferential-stress resisting metal band encircling, and in contact with, the lower portion of the Wall, with the thickness of the bottom portion of the wall 4being less than required to meet circumferential-stress requirements predetermined for the maximum load to be stored in the tank without the circumferential-stress resisting metal band.

The invention will be described further in conjunction with the attached drawings in which:

FIGURE 1 is an isometric view of a vertical cylindrical storage tank having a narrow circumferential-stress resisting metal band encircling the lower portion of the tank wall;

FIGURE 2 is a sectional view of a tank wall of a 200 feet diameter tank showing the height and thickness of the plates in the courses of a banded tank with the horizontal dimension in expanded scale for clarity;

FIGURE 3 is a sectional view of a tank wall of a 200 feet diameter tank constructed according to this invention;

FIGURE 4 is a vertical sectional view of the lower part of the tank wall of FIGURES;

FIGURE 5 is a sectional view of a tank wall of an unbanded 350 feet diameter tank;

FIGURE y6 is a sectional view of a tank wall of a 350 feet diameter tank constructed according to this invention;

FIGURE 7 is a vertical sectional view of the lower part of the tank wall of FIGURE 6;

FIGURE 8 is a sectional view of a tank wall showing a band in the form of a T FIGURE 9 is a sectional view of a tank wall showing a band fabricated of two angles and a metal strip or plate; and

FIGURE l0 is a sectional view of a tank wall showing a band made of a plurality of metal strips wrapped tightly around the tank.

So far as is practical the same numbers will be used to identify the same or similar parts or elements in the drawings.

The wall structures of FIGURES 2, 3, 5 and 6 have been drawn with an expanded horizontal scale compared to the vertical scale in order to show more clearly the relative thickness of the tank wall structures.

The drawings show in FIGURE 1 a vertical cylindrical storage tank 10 according toythis invention. The tank has a circular metal bottom 11 which extends beyond the lower edge of the wall 12. The wall 12 comprises a plurality of metal rings or courses made of metal plates. The lower portion of wall 12 has a circumferentialstress resisting metal band 13 positioned in contact with the wall. The use of metal band 13 permits the use of thinner metal plates for one or more of the lower courses as will be shown hereinafter. The tank is shown covered with a cone roof 14 although other types of roofs, including a floating roof, can be used since the roof structure is not directly involved in the essence of the invention.

The tank wall structure of FIGURE 2 is for an unbanded 200 feet diameter, 64 feet high cylindrical tank using conventional design rules in which the design pressure is calculated one foot above the bottom of the course. The tank is designed to withstand stresses that would be induced in the tank if filled with water. The steel plates of each course are 8 feet high and each course is of decrea-sing thickness from the bottom course to the top course as shown by the dimensions in the drawings. The thickness of each course is determined by the allowable stress of the material used, the liquid to be stored in the tank, the liquid height at the design level and the diameter of the tank although the uppermost course has a minimum thickness required for general stability of the tank shell. The dimensions given in FIGURE 2 are those required according to the American Petroleum Institute Standard 650, Third Edition, July 1966, page 13 Welded Steel Tanks for Oil Storage. The lowermost course or ring 20 requires a thickness of 1.835 inches while the next higher course 21 requires a thickness of 1.602 inches to meet the specified standard rules. However, the maximum thickness permitted by the standard is one and one-half inches.

The shell structure of FIGURES 3 and 4 illustrates one embodiment of the invention as applied to the same size tank, designed to store the same liquid and meet the same stress requirements as the tank of FIGURE 2. The shell or wall of FIGURES 3 and 4 has a circumferential steel band 30 placed around the lowermost course 31. By use of such a band the thickness of the course 31 can be 1.50 inches, instead of 1.835 inches for the same course 20, in FIGURE 2. In addition the second course 32 of the wall structure of FIGURES 3 and 4 can be 1.50 inches instead of 1.602 inches as needed for the same course 21 in the 4structure of FIGURE 2. The higher courses have the same thicknesses as in FIGURE 2. Since the band 30 permits the use of thinner plates in the courses 31 and 32 there is a significant saving in the cost of the plates, welding and weight. By carefully sizing and positioning the band 30 a very substantial savings can be realized. FIGURE 4 shows that a band 1.50 inches thick and 1 foot 5 inches high positioned around the course 31 with the lower edge of the band located 5 feet 7 inches above the tank bottom 11 can be used in conjunction with the thinner plates 31 and 32. The weight of the two lower courses 31 and 32 plus the band 30 would be about 35,000 pounds less than the weight of courses 20 and 21 in FIGURE 2. Nevertheless, such a wall structure can withstand the same stresses as the wall structure of FIGURE 2. The weight of the two 1.5 inches courses 31 and 32 is about 615,000 pounds and the weight of the band is about 55,000 pounds for a total weight of 670,000 pounds for the two courses and the band. The 1.835 inches course 20 of FIGURE 2 weighs about 376,000 pounds and the 1.602 inches course 21 weighs about 329,000 pounds for a total of 705,000 pounds. The weight of the band is thus about 39% less than the difference in weight between the two lower courses of the wall structures of FIGURES 2 and 3 as shown by the calculation:

Perent= 90,000

An unbanded tank structure using conventional design rules for a 350 feet diameter, 60 feet high steel tank is shown in FIGURE 5. The two lower courses S0 and 51 are 10 feet high and the upper live courses are each 8 feet high. The dimensions given in FIGURE 5 are designed to meet the API-650 Appendix G dated December 1967, requirements for a tank of this size using water as the stored product for design purposes.

FIGURE 6 shows a banded tank, according to this invention, having a 350 feet diameter, a 60 feet height and courses `of the same heights as used in the tank of FIG- URE 5. The tank of FIGURE 6 is also designed for stresses induced by water as the stored product. By using a circumferentially positioned steel band 60 around the upper part of the irst course 61 as shown in FIGURE 6, the thickness of this course can be made 1.386 inches, instead of 1.918 inches as for course 50 in FIGURE 5, and the next course 62 can be made 1.386 inches instead of 1.486 inches as required for course 51 in the tank wall structure of FIGURE 5. The other courses in the wall of FIGURE 6 have the same thicknesses as those in FIGURE 5. FIGURE 7 shows that band 60 can be 1.375 inches thick and 2 feet 3 inches high with the bottom edge of the band placed 6 feet 4 inches above the bottom 11. The weight of courses 50 and 51 and band 60 of the wall structure of FIGURES 6 and 7, when made of steel, lwill be about 144,000 pounds less than the wall shown in FIGURE 5. The weight of the 1.918 inches course 50 in FIGURE 5 is about 860,000 pounds and the 1.486 inches course 51 is about 667,000 pounds, giving a total of 1,527,000 pounds for these two courses. The 1.386 inches courses 61 and 62 in FIGURE 6 together weigh about 1,244,000 pounds and the band weighs 139,000 pounds giving a total weight of 1,383,000 pounds for courses 61 and 62 and the band. The weight of the band is thus about 51% less than the difference in weight between the two lower courses of the wall structures of FIGURES 5 and 6 as shown by the calculation:

Percent= X 100 Per0ent=50-9 The particular shape and construction of the circumferential band is not limited since various types of bands can be used as shown in FIGURES 8-10. The band 80 shown in FIGURE 8 is T shaped and is fabricated using a horizontal plate 81 welded at one edge to course 83 and at its other end to vertical plate 82. In FIGURE 9 a band is shown fabricated of two similar angles 91 welded both to course 93 and to plate 92. The band 100 of FIG- URE 10 is composed of a plurality of strips 101 which are wrapped tightly around course 102.

Although the particular shape of the band is not critical it is important to position the band in a location where it will appropriately resist the stresses exerted against the tank shell when loaded. Furthermore, the cross-sectional area of the band must -be adequate to withstand the stresses to which it will be subjected when used with the thinner lower or bottom portion of the tank shell or wall. By bottom portion is meant that portion of the wall or shell of uniform thickness extending from the tank bottom upwardly to a change in thickness of the wall or shell such as at a course seam or horizontal girth Weld. The bottom portion in some instances will be the lowermost course and in other instances will be the lowermost two, three or more courses depending on the height of each course.

In producing a tank shell of steel or aluminum according to this invention with the lower portion of the tank shell extending from the tank or shell bottom to a distance kH upward on the shell, designed for an allowable stress S, a uniform thickness of at least t, having a metal band which reduces the circumferential maximum average stress to a value not exceeding S, the metal lband has an area of A, the distance from the centroid of the band to the bottom of the tank is about L, the allowable average circumferential stress in the band is s, and the tensile force in the band when the tank is empty is F, the following formulas and values can be used:

L is not greater than (3.77-4.06/ b)H b k L/ H 1:1285 HNI-t r is the radius of the tank H is the elective storage height of the shell A is not greater than (0.0614-321/ b)'yH2kr/S where fy is the density of the material to be stored in the tank.

The thickness t of the lowest portion of the tank can be reduced without exceeding the allowable average circumferential-stress S by placing a metal reinforcingV ring around the tank so long asthe distance L from the lbottom of the tank to the centroid of the reinforcing band is not greater than (3.77-'4.06/b)H/b and k L/H, where b=1.2s5 H/vi.

The total quantity of material needed to construct the tank can be reduced -without exceeding the allowable average circumferential stresses S or s by placing a rein- 2 forcing band around the tank so long as the area A of the band does not exceed (0.061-|3.21/b)fyH2kr/S and @5, where b=1.2s5 11A/7.

With no prestress (F :0) and with the allowable average circumferential stresses of tne lower portion of the tank and the reinforcing band equal (S=s), the quantity of material needed to construct the tank with a given kH without exceeding the allowable stress S will be minimal and b1=1.474+1.293 E W2 5.793 2.322 M 1 -bl +-b12 A=27HT (o.o695+13.6/b)

CAD

where Maen/6311+167@ Note l: While k must be this large to 4be mathematically correct, practically it may be somewhat smaller without introducing an error greater than normal engineering tolerances.

Although the tank structure of this invention is particularly suitable for construction of tanks from steel and especially steel tanks having at least a feet diameter, using a steel band, it is also within the invention to make the tank and band of other metals, particularly aluminum.

Additional advantages can be realized from the invention by prestressing the band. Doing this will permit even further reduction in the plate thickness used in the lower portion of the tank shell or wall. Thus, by prestressing to an allowable circumferential stress in the band of 80,000 p.s.i., the area of the band 30 in FIGURE 4 can be reduced from 25.5 sq. in. to 17.8 sq. in. and the thickness of the plates 31 and 32 can be reduced from 1.50 inches to 1.448 inches.

What is claimed is:

1. A storage tank substantially smooth-surfaced in appearance externally having:

a circular bottom and a Vertical self-supporting circular shell extending around the bottom and joined thereto;

the shell comprising a series of metal courses or rings of the same diameter positioned vertically one on top of the other with the lower edge of upper courses set on, and joined to, the upper edge of the lower courses;

each course being of uniform thickness for its height and circumference;

a narrow circumferential-stress resisting metal band encircling, and in contact with, a lower portion of the shell;

the thickness of the band-encircled lower portion of the shell being less than required to meet circumferential-stress requirements predetermined for the maximum load to be stored in the tank without the circumferential-stress resisting metal band;

said lower portion of the shell comprising at least a course around which the band is positioned;

each shell course being thinner than the course below it Vexcept that each course in the said lower portion of the shell is no thicker than any course below it; and

said shell is devoid of axially positioned stiffeners placed around the shell for providing support.

2. A storage tank according to claim 1 in which the lower portion comprises at least the bottom course and the metal band encircles the bottom course.

3. A storage tank according to claim 1 in which the weight of the metal band is at least 20% less than the difference between the weight of the shell employing the band and the weight of a shell of the same metal without the band when both shells are designed to withstand an equivalent pre/determined load within the same circumferential-stress requirement.

4. A storage tank according to claim 1 in which the lower portion comprises at least the two lowermost courses and the metal band encircles one of these courses.

5. A storage tank according to claim 4 in which the thickness of the two lowermost courses is the same.

6. A storage tank according to claim 4 in which the metal band encircles the bottom course.

7. A storage tank according to claim 1 having at least eight courses, in which the lower portion comprises at least the two lowermost courses, the band encircles the bottom course and the weight of the metal band is at least less than the difference between the weight of the two lowest courses of a tank employing the band and the weight of these lowest two courses of a tank of the same metal without the band when both shells are designed to withstand an equivalent predetermined load within the same circumferential-stress requirement.

8. A storage tank substantially smooth-surfaced in appearance externally having:

a circular bottom and a vertical self-supporting circular shell extending around the bottom and joined thereto;

the shell comprising a series of metal courses or rings of the same diameter positioned vertically one on top of the other with the lower edge of upper courses set on the upper edge of, and joined to, the lower courses;

each course being of uniform thickness for its height and circumference;

a narrow circumferential-stress resisting metal band encircling, and in Contact with, the lower portion of the shell;

the thickness of the band encircled lower portion of the shell being less than required to meet circumferential-stress requirements predetermined for the maximum load to be stored in the tank without the circumferential-stress resisting metal band;

each shell course which is governed by stress being thinner than the course below it except that each course in the said lower portion of the shell is no thicker than any course below it;

said shell is devoid of axially positioned stiffeners placed around the shell for providing support; and

said lower portion of the tank shell being that portion of the shell measured from the shell bottom to a distance kH upward on the shell, has an allowable stress S, a uniform thickness of at least t, is reinforced with a metal band which reduces the circumferential maximum average stress to a value not exceeding S, the metal band has an area A, the distance from the centroid of the band to the bottom of the tank is about L, the allowable average circumferential-stress in the band is s, wherein L is not greater than (3.77-4-06/b) 1% 1 1.285 HNE r is the radius of the tank H is the effective storage height ofthe shell A is not greater than (0.06l-l-3.2l/b)q/H2kr/S where 'y is the density of the material to be stored in the tank.

9. A storage tank according to claim 8 in which the lower portion 4of the shell consists of the first, or lowermost, course.

10. A storage tank according to claim 8 in which the lower portion of the shell consists of the two lowermost courses.

11. A storage tank according to claim 8 made of steel and having a diameter of at least feet.

12. A storage tank according to claim 8 in which the weight of the metal band is at least 20% less than the difference between the weight of the shell employing the band and the weight of a shell of the same metal without the band when -both shells are designed to withstand an equivalent predetermined load within the same circumferential-stress requirement without the band.

References Cited UNITED STATES PATENTS 532,9l3 l/l895 Schaefer 220--71 X 1,966,244 7/1934 Hansen 220-5 2,237,308 4/ 1941 Larson.

FOREIGN PATENTS 554,684 7/ 1943 Great Britain.

GEORGE E. LOWRANCE, Primary Examiner 

